Ultrasonic surgical instruments are useful surgical instruments for performing certain medical and surgical procedures. Generally, an ultrasonic surgical tool includes a handpiece that contains at least one piezoelectric driver. A tip is mechanically coupled to the driver and extends forward from the housing or shell in which the driver is disposed. The tip has a head. The head is provided with features, often teeth or flutes, dimensioned to accomplish a specific medical/surgical task. The handpiece is part of an ultrasonic tool system. The system also includes a control console. The control console supplies an AC drive signal to the driver. Upon the application of the drive signal to the driver, the driver cyclically expands and contracts. The expansion/contraction of the driver induces a like movement in the tip and, more particularly, the head of the tip. When the tip so moves, the tip is considered to be vibrating. The vibrating head of the tip is applied against tissue to perform a specific surgical or medical task. For example, some tip heads are applied against hard tissue. One form of hard tissue is bone. When this type of tip head is vibrated, the back and forth vibrations of the tip head remove, saw, the adjacent hard tissue. Other tip heads are designed to be placed against soft tissue. When this tip head vibrates the teeth often remove the tissue by a cutting action. Other ultrasonic tips remove tissue by inducing cavitation in the tissue and surrounding fluid. Cavitation occurs as a result of the tip head moving back and forth. Specifically, as a result of these vibrations, small cavities form in the fluid located immediately adjacent the tissue. These cavities are very small zones of extremely low pressure. A pressure differential develops between contents of the cells forming the tissue and these cavities. Owing to the magnitude of this pressure differential, the cell walls burst. The bursting of these cell walls, removes, ablates, the cells forming the tissue.
The head of an ultrasonic tip is often relatively small. Some heads have diameters of less than 1.0 cm. An ultrasonic tool essentially only removes the tissue adjacent to where the head is applied. Owing to the relative small surface area of their heads, ultrasonic handpieces have proven to be useful tools for precisely removing both hard and soft tissue.
Some ultrasonic tips are provided with a through bore. Simultaneously with the application of a drive signal to this type of tip, a suction is drawn through the bore. The suction draws away the debris created by tissue removal process. This is why some ultrasonic tools are sometimes called ultrasonic aspirators.
For an ultrasonic surgical instrument, sometimes called a handpiece or a tool, to efficiently function, a drive signal having the appropriate characteristics should be applied to the tool. If the drive signal does not have the appropriate characteristics, the tip head may undergo vibrations of less than optimal amplitude. If the handpiece is in this state, the ability of the handpiece to, at a given instant, remove tissue may be appreciably reduced.
One means of ensuring that an ultrasonic handpiece operates efficiently is to apply a drive signal to the handpiece that is at the resonant frequency of the handpiece. When the drive signal is at a given voltage or current, the application of the drive signal at the resonant frequency induces vibrations in the tip that are large in amplitude in comparison to the application of the same voltage at a frequency that is off resonance.
Still other ultrasonic tool systems are designed to apply a drive signal at the anti-resonant frequency of the handpiece. The anti-resonant frequency may be a frequency at which the handpiece would have the highest impedance. Sometimes it is desirable to apply a drive signal that is at a frequency somewhere between the resonant and anti-resonant frequencies of the handpiece.
Further, the amplitude of the tip vibrations is also related to the potential, the voltage, of the drive signal. Generally, the amplitude of the tip vibrations is proportional to the voltage of the drive signal. There is however, typically a voltage that, once exceeded, will not result in an increase in the amplitude of the tip vibrations.
Internal to the console are the components that generate the drive signal. Generally, the components integral with the console can be broken down into four main sub-assemblies. A first sub-assembly includes the sensing components. These components monitor the characteristics of the drive signal sourced to the handpiece. An input/output assembly serves as an interface through which the surgeon enters commands regarding the characteristics of the drive signal that is to be applied to the handpiece and over which information regarding the status of the operation of the system is displayed. The third assembly is the controller. The controller, based on the user-entered commands and the signals from the sensing components, generates control signals. The controller also generates information that is presented on the input/output assembly.
The control signals generated by the controller are applied to the fourth sub-assembly of console components, the amplifier. This is because, owing to the limitations of components forming the controller, the control signals typically have potentials of 10 Volts or less and often 5 Volts or less. For the drive signal to induce the desired contractions and expansions of the transducers, the signal typically needs to have a potential of at least 500 volts and often 1000 volts. The amplifiers of many consoles amplify the control signal so the output signal produced by the amplifier is at the potential at which the output signal can function as the drive signal applied to the handpiece.
Applicant's SONOPET® Ultrasonic Aspirator includes a console with components designed to generate and apply a variable drive signal to the attached handpiece. Internal to the console is a resonance circuit. At the time of manufacture of the console, the inductance and capacitance of this resonance circuit are set as a function of the impedance of the specific handpiece with which the console is intended to be used. The characteristics of the drive signal output by the console are set as a function of the voltage across this impedance circuit.
The control consoles provided with many ultrasonic tool systems include amplifiers capable of outputting drive signals that, over narrow frequency ranges, foster the desired handpiece driver expansions and contractions. For example, some control consoles output drive signals that have a frequency between 25.2 kHz and 25.6 kHz. This type of control console works well with a handpiece that includes drivers designed for actuation by drive signals that have a frequency within this range of frequencies. If a handpiece with drivers designed to receive drive signals over a different frequency range is attached to the console, the responsiveness of the handpiece to the out of range drive signals will be less than optimal.
As a consequence of this limitation, if a facility wants to use ultrasonic handpieces to which appreciably different drive signals are applied, it may be necessary to provide plural control consoles. Specifically, one console would be used to provide drive signals to handpieces to which drive signals having a first set of characteristics are applied. A second console is used to provide drive signals to the handpieces to which drive signals having a second set of characteristics are applied. Having to provide these plural consoles that differ only in the form of the drive signals they generate adds to the expense and administrative burden of operating the facility using this equipment.
Further, a console may not generate the optimal drive signals for some operating states even when the console is generating the signals within the intended frequency range of drive signals the console is designed to produce. This is because at one or both ends of the range of voltages of the drive signals the console is intended to produce, the amplifier internal to the console may not provide a linear response to input signals used to establish the voltage of the drive signals.
In addition, some tips are designed to, when actuated, vibrate with a motion that is combination of two distinct motions. For example, some tips are designed to engage in vibrational motion that is the sum of two components. The first component is the longitudinal vibration. This is the back and forth vibration along the longitudinal axis of the tip. The second component is the rotational or torsional vibration. This motion is a back and forth rotational motion around the longitudinal axis of the tip. Generally, a tip able to vibrate simultaneously in two modes is referred to as a tip able to engage in a bi-modal vibration. A tip designed to vibrate simultaneously in three or more modes is referred to as a tip able to engage in multi-modal vibration.
For a tip to engage in bi-modal or multi-modal vibrations, it is desirable to apply a drive signal to the tip that is a composite of the signals best suited to drive the tip in each of its vibratory modes. Often these signals are at different frequencies. A console that can only generate drive signals over a narrow range of frequencies is often for unsuitable for generating a drive signal that is composite of components that have frequencies that may differ by 1,000 Hz or more.